Butt weld it in. Cut out the rust, get new steel good steel from a donor car. Spend time cutting and trimming the replacement section so it fits perfectly, a nice 1mm gap or less all the way around will be perfect. Then tack the section in at regular intervals, tweak it as you go so it sits correctly. When happy with that seam weld in about 25mm stretches in alternative locations around the section, allowing it to cool between welds to prevent distortion. Tidy up the welds afterwards with a grinder/flap disc, then degrease, Metal Ready it, then zinc or etch primer it before top coating.
NVQ Level 1, is that good or bad?
